accept(2)                                               System Calls Manual                                               accept(2)

       accept, accept4 - accept a connection on a socket

       Standard C library (libc, -lc)

       #include <sys/socket.h>

       int accept(int sockfd, struct sockaddr *_Nullable restrict addr,
                  socklen_t *_Nullable restrict addrlen);

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <sys/socket.h>

       int accept4(int sockfd, struct sockaddr *_Nullable restrict addr,
                  socklen_t *_Nullable restrict addrlen, int flags);

       The  accept()  system  call is used with connection-based socket types (SOCK_STREAM, SOCK_SEQPACKET).  It extracts the first
       connection request on the queue of pending connections for the listening socket, sockfd, creates a new connected socket, and
       returns a new file descriptor referring to that socket.  The newly created socket is not in the listening state.  The origi‐
       nal socket sockfd is unaffected by this call.

       The argument sockfd is a socket that has been created with socket(2), bound to a local address with bind(2), and is  listen‐
       ing for connections after a listen(2).

       The argument addr is a pointer to a sockaddr structure.  This structure is filled in with the address of the peer socket, as
       known to the communications layer.  The exact format of the address returned addr is determined by the socket's address fam‐
       ily  (see  socket(2) and the respective protocol man pages).  When addr is NULL, nothing is filled in; in this case, addrlen
       is not used, and should also be NULL.

       The addrlen argument is a value-result argument: the caller must initialize it to contain the size (in bytes) of the  struc‐
       ture pointed to by addr; on return it will contain the actual size of the peer address.

       The  returned  address  is  truncated if the buffer provided is too small; in this case, addrlen will return a value greater
       than was supplied to the call.

       If no pending connections are present on the queue, and the socket is not marked as nonblocking, accept() blocks the  caller
       until a connection is present.  If the socket is marked nonblocking and no pending connections are present on the queue, ac‐
       cept() fails with the error EAGAIN or EWOULDBLOCK.

       In order to be notified of incoming connections on a socket, you can use select(2), poll(2), or epoll(7).  A readable  event
       will  be  delivered  when  a new connection is attempted and you may then call accept() to get a socket for that connection.
       Alternatively, you can set the socket to deliver SIGIO when activity occurs on a socket; see socket(7) for details.

       If flags is 0, then accept4() is the same as accept().  The following values can be bitwise ORed in flags to obtain  differ‐
       ent behavior:

       SOCK_NONBLOCK   Set  the  O_NONBLOCK file status flag on the open file description (see open(2)) referred to by the new file
                       descriptor.  Using this flag saves extra calls to fcntl(2) to achieve the same result.

       SOCK_CLOEXEC    Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor.  See the description  of  the  O_CLOEXEC
                       flag in open(2) for reasons why this may be useful.

       On  success,  these  system calls return a file descriptor for the accepted socket (a nonnegative integer).  On error, -1 is
       returned, errno is set to indicate the error, and addrlen is left unchanged.

   Error handling
       Linux accept() (and accept4()) passes already-pending network errors on the new socket as an error code from accept().  This
       behavior  differs  from  other BSD socket implementations.  For reliable operation the application should detect the network
       errors defined for the protocol after accept() and treat them like EAGAIN by retrying.  In the case  of  TCP/IP,  these  are

              The  socket is marked nonblocking and no connections are present to be accepted.  POSIX.1-2001 and POSIX.1-2008 allow
              either error to be returned for this case, and do not require these constants to have the same value, so  a  portable
              application should check for both possibilities.

       EBADF  sockfd is not an open file descriptor.

              A connection has been aborted.

       EFAULT The addr argument is not in a writable part of the user address space.

       EINTR  The system call was interrupted by a signal that was caught before a valid connection arrived; see signal(7).

       EINVAL Socket is not listening for connections, or addrlen is invalid (e.g., is negative).

       EINVAL (accept4()) invalid value in flags.

       EMFILE The per-process limit on the number of open file descriptors has been reached.

       ENFILE The system-wide limit on the total number of open files has been reached.

              Not  enough  free memory.  This often means that the memory allocation is limited by the socket buffer limits, not by
              the system memory.

              The file descriptor sockfd does not refer to a socket.

              The referenced socket is not of type SOCK_STREAM.

       EPERM  Firewall rules forbid connection.

       EPROTO Protocol error.

       In addition, network errors for the new socket and as defined for the protocol may be returned.  Various Linux  kernels  can
       return  other errors such as ENOSR, ESOCKTNOSUPPORT, EPROTONOSUPPORT, ETIMEDOUT.  The value ERESTARTSYS may be seen during a

       The accept4() system call is available starting with Linux 2.6.28; support in glibc is available starting with glibc 2.10.

       accept(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (accept() first appeared in 4.2BSD).

       accept4() is a nonstandard Linux extension.

       On Linux, the new socket returned by accept() does not inherit file status flags such as O_NONBLOCK  and  O_ASYNC  from  the
       listening  socket.   This behavior differs from the canonical BSD sockets implementation.  Portable programs should not rely
       on inheritance or noninheritance of file status flags and always explicitly set all required flags on  the  socket  returned
       from accept().

       There  may  not  always be a connection waiting after a SIGIO is delivered or select(2), poll(2), or epoll(7) return a read‐
       ability event because the connection might have been removed by an asynchronous network error or another thread  before  ac‐
       cept()  is called.  If this happens, then the call will block waiting for the next connection to arrive.  To ensure that ac‐
       cept() never blocks, the passed socket sockfd needs to have the O_NONBLOCK flag set (see socket(7)).

       For certain protocols which require an explicit confirmation, such as DECnet, accept() can be thought of as merely dequeuing
       the next connection request and not implying confirmation.  Confirmation can be implied by a normal read or write on the new
       file descriptor, and rejection can be implied by closing the new socket.  Currently, only  DECnet  has  these  semantics  on

   The socklen_t type
       In  the  original  BSD sockets implementation (and on other older systems) the third argument of accept() was declared as an
       int *.  A POSIX.1g draft standard wanted to change it into a size_t *C; later POSIX standards and glibc 2.x have socklen_t *

       See bind(2).

       bind(2), connect(2), listen(2), select(2), socket(2), socket(7)

Linux man-pages 6.03                                         2022-12-04                                                   accept(2)